Polycrystalline silicon resistors, also called polysilicon or polySi resistors, have been used in the electronic circuit industry for many years. Depending upon their doping and doping levels, p+, n+, p− and n− polysilicon resistors can be fabricated. Resistors including p+ polysilicon are extremely advantageous for use in analog circuit designs due to their desirable figures of merits. Typically, p+ polysilicon resistors are used in analog and mixed signal circuit designs because they can provide precise matching of subsequent resistors, a low temperature coefficient of resistance, a low voltage coefficient of resistance, and a low parasitic capacitance.
Although polysilicon resistors are widely used in analog circuit designs, such resistors generally have high sheet resistance tolerances ranging from 15-20%. This means the sheet resistance changes by +/−15 to 20%. In current analog and mixed signal applications, and in order to meet stringent circuit performance requirements, circuit designers are demanding lower tolerances in polysilicon resistors.
In the current state of the art, polysilicon resistors are fabricated by ion implanting dopants into a polysilicon layer during the source/drain (S/D) implant step and/or emitter implant step. The implanted dopants in the various regions are then activated utilizing a rapid thermal anneal process. Next, a dielectric layer such as a nitride is applied to the body of the polysilicon resistor so as to protect the body of the polysilicon resistor from being silicided in a subsequent silicidation step.
The ends of the polysilicon resistors are then typically exposed and silicided by employing a conventional silicidation process that includes depositing a metal atop the exposed polysilicon end portions and annealing. A single or two-step anneal process may be used in forming the silicide depending on the type of metal that is deposited. The two-step anneal typically includes a silicidation formation anneal and a silicidation transformation anneal. During the annealing step, the resistance of the polySi resistor typically changes such that the resistance value no longer meets a predetermined and required resistance value.
Other thermal cycles besides the silicidation anneals (e.g., formation anneal and transformation anneal) can also adversely affect the resistance of a polysilicon resistor. For example, the anneal used to activate dopants within the source/drain regions, the temperature of various materials being deposited atop the doped polysilicon layer, and gate sidewalls oxidations can also change the resistance value such that it no longer meets a predetermined specification.
Co-assigned and co-pending U.S. application Ser. No. 10/605,439, filed Sep. 30, 2003 describes processes for fabricating high precision polysilicon resistors which can avoid some of the problems, particularly the change in resistance, that are caused by the thermal cycling mentioned above. In particular, and in an embodiment of the '439 application, the precision resistor is formed by performing a rapid thermal anneal for an emitter/FET activation process on a wafer or chip having a partially formed polysilicon resistor having a polysilicon layer, depositing a protective layer over the polysilicon layer to protect the polysilicon layer against subsequent silicide processing, ion implanting a dopant into the polysilicon layer through the protective layer, and performing silicide processing to form the precision polysilicon resistor.
With respect to this embodiment disclosed in the '439 application, the prior art technique requires that the implanted dopants within the polysilicon layer be activated by the silicidation anneal. The activation of the dopants within the polysilicon layer is possible using the second method disclosed in the '439 application when the silicide anneal is performed at temperatures greater than 700° C. However, for low temperature silicidation processes in which the anneal temperature is less than 700° C., preferably less than 400° C., the method disclosed in the '439 application does not work very effectively since the low silicidation temperatures are not capable of fully activating the dopants within the polysilicon layer. Hence, the resultant resistor would exhibit some resistance variation caused by further thermal processes.
In view of the above drawbacks with the prior art processes of fabricating Si-containing resistors, e.g., polysilicon or polySiGe, particularly the difficulties in controlling the resistance of resistors, there is a need for developing a new and improved method in which resistors, including polySi or polySiGe resistors, can be fabricated that exhibit less sheet resistance variation than compared to conventional polysilicon resistors.